One of the problems faced by the perfumery industry lies in the relatively rapid loss of the olfactive benefit provided by odoriferous compounds due to their volatility, particularly that of “top-notes”. This problem is generally tackled using a delivery system, e.g. capsules containing a perfume, to release the fragrance in a controlled manner Aminoplast microcapsules formed of a melamine-formaldehyde resin are frequently used to encapsulate hydrophobic actives, thus protecting said actives and providing their controlled release.
However, capsules such as aminoplast ones suffer from stability problems when used in consumer products comprising surfactants, such as perfumery consumer products, especially after prolonged storage at elevated temperatures. In such products, the encapsulated active tends to leak out of the capsule, even though the capsule wall remains intact, by diffusion through the wall due to the presence of surfactants that are able to solubilise the encapsulated active in the product base. The leakage phenomenon reduces the efficiency of the capsules in protecting the active and providing its controlled release. This is especially disadvantageous when the active is a volatile ingredient such as a perfume.
Diverse technologies have been developed to improve the stability of aminoplast capsules, i.e. reducing perfume leakage in perfumery consumer products. In EP1767185, the composition of the encapsulated material is specifically designed to avoid leakage. Another approach described in the prior art for example in US2005/0112152 and US2005/0153135, consists in further diluting the perfume oil into an equal amount of very hydrophobic solvent with a log P>5.5 to reduce perfume leakage. However, this drastically reduces the pure perfume oil payload in the capsule. US2006/0248665 also addresses the reduction of perfume leakage rate by catalysing the curing cross-linking reactions in the wall by acid, metal salt or high temperature catalysis.
Alternatively, some prior art documents such as WO2004/016234 disclose coating of the capsules with an additional layer or shell (two shells systems).
Whatever solutions have been proposed to address the problem of perfume leakage, all these prior arts have in common the fact that the ratio between the quantity of aminoplast resin (melamine-formaldehyde) used to make the capsule wall to the quantity of encapsulated perfume oil in the hydrophobic core of the capsule is relatively high, as it is well known heretofore to those skilled in the art that the more melamine-formaldehyde resin is used to make the capsule walls, the better the storage stability of the capsules and therefore the lower the perfume leakage in aqueous surfactant-rich consumer products like fabric-softener, liquid detergents, shampoo, hair conditioners or yet shower gels. The ratio of pure melamine-formaldehyde resin used to make the wall to the perfume oil payload of the capsules typically varies between 0.12 and 0.20 as exemplified in the various patent applications mentioned above.
However, while the use of a large amount of aminoplast resin has always been described as helping stabilization of the capsule and reducing perfume leakage (by making the wall thicker), it also has a negative effect on olfactive performance of the capsule, making it more difficult to break to release its perfume payload thus reducing the end-consumer benefit.
More recently, it has been described in WO2008/098387A1 that the use of aromatic polyols like resorcinol in the water phase to cross-link the melamine-formaldehyde wall would stabilize it and reduce perfume leakage upon storage. However that prior art still teaches that the lower the ratio of melamine-formaldehyde resin used to the perfume oil, the higher the perfume leakage upon storage in fabric-softener. In particular, even for the more stable capsules described with resorcinol, reducing the ratio from 0.15 to 0.09 leads to close to doubling perfume leakage upon storage, thus limiting the expectation of a benefit when further reducing the ratio of melamine-formaldehyde resin to the perfume oil. WO2013/092375 A1 from the applicant describes another way to achieve some of these goals by stabilizing the aminoplast microcapsules with a polyisocyanate cross-linker pre-dissolved in small quantity in the perfume oil core prior to encapsulation.
Despite those teachings, there is still a need to improve aminoplast-based perfume delivery systems. In particular it would be advantageous to find a capsule encapsulating a perfume oil which would combine good storage stability of the capsules with a reduced perfume leakage in aqueous surfactant-rich consumer products together with improved olfactive performance/ease of breaking during gentle handling and when rubbing. This olfactive performance translates in the perfume intensity perceived e.g. on dry fabrics.